When pulling a webpage from an URL, Facebook caches its content for later. It means that if Facebook once pulls data from your site, and than your site changes (for example your og:image tags change), Facebook will not change the image, because it already has everything cached on its side.
Try visiting Facebook Debugger page and reviewing what Facebook “sees”. Using this tool forces Facebook to refresh data from the given URL. This is the best way to debug it.
When you make sure Facebook “sees” proper og:image tags, then make sure the images in the og:image are of proper minimum width/height (it’s all explained in the fb debugger). Facebook will not use an image from og:image if it does not meet FB’s criteria.
I strongly recommend this tool whenever there are any issues with content from your site on FB.